#include <vector>
#include <set>
#include <iostream>
#include <unordered_map>
#include <numeric>

using namespace std;

char findTheDifference(string s, string t) {
    vector<int> num(122-97+1, 0);
    for(int i = 0; i < s.size(); i++){
        num[int(s[i])-97]++;
    }
    for(int i = 0; i < t.size(); i++){
        if(--num[int(t[i])-97] == -1){
            return t[i];
        }
    }
    return ' ';
}

int main(){
    string s = "abcd", t = "abcde";
    char ret = findTheDifference(s, t);
    cout << ret << endl;
    return 0;
}